1. Field of the Invention
The present invention relates in general to gaseous fuel burners. More specifically the invention relates to energy efficient burning of fuel using such burners.
2. Related Art
Oxy-fuel burners and technologies are being used more and more in high temperature processes such as, glass manufacturing, incineration of wastes, steel reheating, aluminum smelting, and iron smelting, for the benefits they provide:
high heat transfer rates; PA1 fuel consumption reductions (energy savings); PA1 reduced volume of flue gas; PA1 reduction of pollutants emission, such as oxides of nitrogen (NOx), carbon monoxide (CO), and particulates. PA1 (a) at least one primary means for transferring heat between a hot flue gas having a hot flue gas temperature, and an initially cold intermediate fluid, the initially cold intermediate fluid having a cold intermediate fluid temperature which is less than the hot flue gas temperature, to create a hot intermediate fluid and to cool the hot flue gas; PA1 (b) one or more oxidant-fuel burners which create the main flow of the hot flue gas, the oxidant-fuel burners being associated with preheater means in which either a fuel or an oxidant is preheated by the hot intermediate fluid, and thus creating a cooled intermediate fluid, and; PA1 (c) transport means for transporting the hot intermediate fluid to at least one of the preheater means. PA1 (a) at least one primary means for transferring heat between a hot flue gas having a hot flue gas temperature, and an initially cold intermediate fluid, the initially cold intermediate fluid having a cold intermediate fluid temperature which is less than the hot flue gas temperature, to create a hot intermediate fluid and to cool the hot flue gas; PA1 (b) one or more oxidant-fuel burners which create the main flow of hot flue gas, the oxidant-fuel burners having a fuel path for a fuel, an oxidant path for an oxidant, and a hot intermediate fluid path, wherein the hot intermediate fluid exchanges heat with either the oxidant, the fuel, or both the oxidant and the fuel, to create a cooled intermediate fluid; and PA1 (c) transport means for transporting the hot intermediate fluid to the oxidant-fuel burners. PA1 a) combusting the fuel with the oxidant in one or more oxidant-fuel burners to create the main flow of hot flue gas; PA1 b) flowing a hot flue gas and an initial intermediate fluid having an initial intermediate fluid temperature through primary means for transferring heat between the hot flue gas and the initial intermediate fluid to create a hot intermediate fluid; PA1 c) transferring heat from the hot intermediate fluid to either the fuel, the oxidant, or both, by flowing the hot intermediate fluid through one or more preheater means in which either the fuel, the oxidant, or both are preheated with the hot intermediate fluid prior to the fuel and the oxidant entering one or more oxidant-fuel burners which create said main flow of hot flue gas. PA1 a) combusting the fuel with the oxidant in one or more oxidant-fuel burners to create the main flow of hot flue gas; PA1 b) flowing a hot flue gas and an initial intermediate fluid, having an initial intermediate fluid temperature, through primary means for transferring heat between the hot flue gas and the initial intermediate fluid to create a hot intermediate fluid; PA1 c) transferring heat from the hot intermediate fluid to either the fuel, the oxidant, or both, by flowing the hot intermediate fluid through one or more oxidant-fuel burners which create the main flow of hot flue gas, the oxidant-fuel burners having a fuel path for a fuel, an oxidant path for an oxidant, and a hot intermediate fluid path, wherein the hot intermediate fluid exchanges heat with either the oxidant, the fuel, or both the oxidant and the fuel, to create a cooled intermediate fluid. PA1 a) creating the main flow of hot flue gas by burning a fuel with an oxidant in a plurality of side-mounted burners, a first portion of the burners mounted on a first side of a furnace, and a second portion of the burners mounted on an opposite side of the furnace; PA1 b) flowing the hot flue gas through a stack at a first temperature (preferably at a temperature ranging from about 1000.degree. C. to about 1700.degree. C.); PA1 c) flowing an initial intermediate fluid having an initial intermediate fluid temperature (preferably air at ambient temperature, about 25.degree. C.), through a primary means for transferring heat (preferably a radiative metallic recuperator) to preheat the initial intermediate fluid (preferably to a temperature ranging from about 500 to about 900.degree. C.) thus creating a hot intermediate fluid; PA1 d) splitting the hot intermediate fluid flow into two streams, a first stream flowing to the first side of the furnace and a second stream flowing to the opposite side of the furnace, wherein on each of said first and opposite sides of the furnace are positions a plurality of oxidant preheaters and a plurality of fuel preheaters, and a plurality of burners, (preferably the number of oxidant preheaters is less than the number of burners and the number of fuel preheaters is less than the number of burners, (preferably the burners are grouped in pairs in order to reduce the number of oxidant and fuel preheaters); PA1 e) flowing each of the first and second flows of hot intermediate fluid through one or more oxidant preheaters (preferably metallic or ceramic) in series, thus creating first and second flows of cooled intermediate fluid and a plurality of heated oxidant streams (the heated oxidant preferably having a temperature ranging from about 400 to about 800.degree. C.); PA1 f) flowing the cooled intermediate fluid through the fuel preheaters also installed in series, thereby creating a cold intermediate fluid and a plurality of heated fuel streams (preferably heating the fuel to a temperature ranging from about 200 to about 300.degree. C.); and PA1 g) splitting the heated oxidant and heated fuel streams to amount of streams equal to the number of burners to combust the fuel in the furnace, thus creating the main flow of hot flue gas. PA1 a) combusting a fuel in a primary oxidant-fuel burner positioned at an end of the end-fired furnace, the primary burner supplying the main part of the energy to a load and creating the main flow of hot flue gas, and one or more additional conventional oxidant-fuel burners positioned generally opposite of the primary burner, for better coverage of a firing zone in the end-fired furnace; PA1 b) flowing the hot flue gas through a stack at a first temperature (preferably at a temperature ranging from about 1000.degree. C. to about 1700.degree. C.); PA1 c) flowing an initial intermediate fluid having an initial intermediate fluid temperature (preferably air at ambient temperature, about 25.degree. C.), through a primary means for transferring heat (preferably a radiative metallic recuperator) to preheat the initial intermediate fluid (preferably to a temperature ranging from about 500 to about 900.degree. C.) thus creating a hot intermediate fluid; PA1 d) flowing the hot intermediate fluid to an oxidant preheater, thus producing a first cooled intermediate fluid and preheated oxidant; PA1 e) flowing the cooled intermediate fluid to a fuel preheater, thus producing a second cooled intermediate fluid and heated fuel, the second cooled intermediate fluid having a temperature lower than the first cooled intermediate fluid, (preferably flowing the second cooled intermediate fluid to the stack at a temperature of about 300.degree. C.); and PA1 f) flowing the heated oxidant and heated fuel streams to the primary oxidant-fuel burner to create the main flow of the hot flue gas leaving the furnace, the conventional burners also contributing to the hot flue gas.
Oxygen used in these high temperature processes can be technically pure oxygen (99.99%) or various grades of industrial oxygen, with purities down to 80%.
Despite the reduction of the flue gas volume that the substitution of combustion with air by combustion with pure oxygen yields, a significant amount of energy is lost in the flue gas, especially for high temperature processes. For example, in an oxy-fuel fired glass furnace where all the fuel is combusted with pure oxygen, and for which the temperature of the flue gas at the furnace exhaust is of the order of 1350.degree. C., typically 30% to 40% of the energy released by the combustion of the fuel is lost in the flue gas. It would be advantageous to recover some of the energy available from the flue gas in order to improve the economics of operating an oxy-fuel fired furnace.
A number of techniques to recover energy from flue gases are available. Those techniques have been proven or described for air-fuel fired furnaces. Similar techniques have yet to be demonstrated for oxy-fuel furnaces, because of difficulties that will become apparent from the following discussion.
One technique consists in using the energy available in the flue gas to preheat and dry out the raw materials before loading them into the furnace. In the case of glass melting, the raw materials consist of recycled glass, commonly referred to as cullet, and other minerals and chemicals in a pulverized form referred to as batch materials that have a relatively high water content. The energy exchange between the flue gas and the raw materials is carried out in a batch/cullet preheater. Such devices are commonly available, for example from Zippe Inc. of Wertheim, Germany. Experience shows that this technology is difficult to operate when the batch represents more than 50% of the raw materials because of a tendency to plug. This limits the applicability of the technique to a limited number of glass melting operations that use a large fraction of cullet. Another drawback of this technique is that the inlet temperature of the flue gas in the materials preheater must be generally kept lower than 600.degree. C. In the case of an oxy-fuel fired furnace where the flue gas is produced at a temperature higher than 1000.degree. C., cooling of the flue gas prior to the materials preheater would be required.
Energy efficiency of air-fuel furnaces is greatly improved if the energy available from the flue gas is used to preheat the combustion air. Recuperators, where some of the heat from the flue gas is transferred to the combustion air in a heat exchanger, and regenerators, where some of the heat from the flue gas is accumulated in a ceramic or refractory material for later preheating of the combustion air, are the most common techniques encountered in the industry for this purpose. Such techniques are difficult to apply in the case of oxy-fuel fired furnaces because of the hazards of handling the extremely reactive hot oxygen.
Thermochemical energy recovery (also known as fuel reforming) is another technique that consists in increasing the heat content of a fuel by reacting it with steam or carbon dioxide or a mixture of the two in a reactor (reformer), and generating a combustible mixture that contains hydrogen (H.sub.2) and carbon monoxide (CO) and has a higher heat content than the initial fuel. The reforming reaction occurs at high temperature (typically 900.degree. C.), is endothermic, and takes advantage of the high temperature of the flue gases to generate the high temperature gases required by the process, and to provide the energy for the reforming reaction. Practically, the fuel consumption in a glass plant is not high enough to provide an economical justification to the high capital cost of installing a fuel reforming system. The complexity of the reformer, and safety constraints linked to handling hot H.sub.2 and CO, are additional drawbacks of this technology. In the case of oxy-fuel furnaces, the energy available from the flue gas is typically not sufficient for reforming all the fuel, and an additional energy source is generally required in addition to the flue as, which adds to the complexity of the apparatus.
Co-generation of power and heat (i.e. the simultaneous generation of electricity and steam using the hot flue gases) is another technique that is available to recover the energy from flue gas, and use it for other purposes than recycling into the furnace. The disadvantage of this approach is that the capital costs tend to be very high. This option is, however, viable for very high heat output furnaces (those which produce greater than 30 megawatts of power).
With stricter environmental regulations, a number of industries are required to install pollution abatement systems. Those devices typically cannot handle the very high temperatures found at the exhaust of an oxy-fuel furnace used for a high temperature process. For instance, at the outlet of an oxy-fuel fired glass tank furnace, the temperature typically ranges from about 1300.degree. C. to about 1450.degree. C. Before the flue gases can be treated by the pollution abatement system (which can be an electrostatic precipitator or a baghouse in the case of cleaning the flue gas from particulate matter) it is highly preferable to cool down the gases. This is generally performed by diluting the gases with ambient air, or spraying of water that vaporizes upon contact with the hot gases, to yield a cooling of the gases, or by a combination of these techniques. Dilution with air increases the amount of gas to be treated by the pollution abatement system, which increases its cost. Water injection elevates the dew point of the gases and forces the pollution abatement device to operate at high temperature. This is especially true for oxy-fuel fired furnaces where the water content of the flue gases can be as high as 60% by volume.
What is needed then is a method and apparatus (or system) which efficiently and at relatively low capital cost recovers at least a portion of the available heat which otherwise is wasted to the atmosphere, particularly in high temperature processes where oxy-fuel burners are employed, and simultaneously cools down the flue gases.